There are no known point sources of mercury (such as industrial or mining sources) to the study streams. Thus, mercury contamination was not acute, but rather was typical of ecosystems that receive mercury predominantly from atmospheric deposition.

Our study did not specifically target streams with high levels of mercury, but included streams with a range of mercury concentrations. Several streams included in the study are listed in their State health department’s fish consumption advisories:
Neither of the two streams in Oregon is included in Oregon’s fish consumption advisory (http://www.oregon.gov/DHS/ph/envtox/fishconsumption.shtml#table ).
The Wisconsin streams fall under that state’s general, statewide fish consumption advisory (http://dnr.wi.gov/fish/consumption/generaladvice.html ).
Two of the three Florida streams (St Marys River and Santa Fe River) are listed in Florida’s fish consumption advisory (http://www.doh.state.fl.us/floridafishadvice/ ).
None of the streams in this study had “do not eat” advisories. Rather, the advisories contain information regarding how much fish eat from the State’s waters in order to limit one’s exposure to mercury. Refer to state-issued fish consumption advisories for specific advice—these are typically available from State health or natural resources departments.

This is the first known study to use multiple media (water, sediment, biota) to evaluate processes affecting mercury cycling and bioaccumulation in streams across a large geographic area during multiple years. Much of the previous research on mercury in aquatic systems has been on lakes or wetlands. Most prior studies of mercury in streams have focused on fewer media, in smaller geographic areas, during shorter time scales.

Our study sites are typical of most aquatic ecosystems in that they that lack direct industrial, mining, or geologic point sources of mercury to the water. Mercury is delivered to the earth’s surface by atmospheric deposition. Sources of mercury to the atmosphere include both industrial and natural emissions. According to the U.S. Environmental Protection Agency’s 1997 Report to Congress (http://www.epa.gov/ttn/oarpg/t3/reports/volume2.pdf), mercury emissions in the United States are largely from combustion of materials that contain mercury, with coal-fired utility boilers and commercial/industrial boilers being the largest sources. Natural sources include volcanoes and degassing of elemental mercury from the oceans.

Methylmercury is an organic form of mercury that is produced largely as a byproduct of natural microbial processes, when inorganic mercury is present. Methylmercury is an extremely toxic form of mercury.
Aqueous methylmercury is efficiently taken up by lower aquatic organisms and is biomagnified in aquatic food chains, that is, concentrations of methylmercury increase at successively higher steps in aquatic food chains, reaching highest levels in top-predator game fish. Methylmercury may reach levels of concern in game fish, prompting fish-consumption advisories to protect human health. As a result, mercury is currently the second leading cause of impaired waters in the United States, accounting for over 9,000 impaired waters. (http://iaspub.epa.gov/waters10/attains_nation_cy.control?p_report_type=T#causes_303d)

Stream water typically carries trace levels of mercury and methylmercury. Median concentrations of mercury in filtered water at our eight study sites ranged from about 0.4 to 4.9 nanograms per liter (parts per trillion). Median concentrations of methylmercury ranged from less than 0.04 to 0.32 nanograms per liter.
Top-predator game fish in our study streams had median methylmercury concentrations ranging from about 0.02 to 1.2 micrograms per gram (parts per million), expressed on a wet weight basis.
On an equal mass basis, the concentration of methylmercury in fish is approximately one million times greater than in stream water.

Mercury in streams is strongly controlled by runoff from the watershed. Elevated runoff after rain and snowmelt tends to carry higher concentrations of mercury, along with higher concentrations of dissolved organic carbon and suspended sediment.
Mercury effectively binds to both dissolved organic carbon and suspended sediment, so processes that enhance transport of these constituents also enhance the transport of mercury.
The abundance of wetlands in a stream’s watershed correlated with mercury and methylmercury levels in stream water. Wetland environments are conducive to methylation of inorganic mercury.

Both the activity of microbes that methylate mercury and the availability of inorganic mercury to those microbes control the formation of methylmercury in streambed sediment. Microbial activity was typically higher in organic-rich sediment. In reducing (anoxic) sediment, inorganic mercury was less available to microbes than in more oxygenated sediment.
Methylmercury concentrations in sediment pore water tended to be higher in streams with higher pore-water dissolved organic carbon concentrations. Similar to the stream-water findings, streams with abundant wetlands in the watershed had high dissolved organic carbon concentrations in sediment pore water.
Urban streams had more total mercury in sediment than nonurban streams. However, in nonurban streams, a higher proportion of total mercury was in the methylmercury form than in urban streams.

Two key lines of evidence point to watershed processes as more important than in-channel processes.
First, stream-reach-integrated measures of total mercury and methylmercury concentrations in sediment, and methylmercury production in sediment, were not correlated to methylmercury concentrations in the stream water. This study was one of the first to assess methylmercury production in streambed sediment.
Second, in streams where methylmercury was frequently detected in stream water, it usually correlated positively with streamflow, dissolved organic carbon, and suspended sediment, pointing to runoff from the watershed as the dominant source of methylmercury to the stream.

Methylmercury is taken up by stream biota primarily through the food they eat. Methylmercury is not readily excreted, so when an organism consumes another organism, it retains and accumulates the mercury in that organism. Predator fish are eating at or near the top of the local food chain, so they acquire all the methylmercury that has been accumulated at lower levels of the food chain. Additionally, predator fish are longer-lived than organisms lower in the food web, so they have more time to accumulate more mercury over their lifespan.

Food chain length and trophic level (position in the food chain) are important controls on methylmercury bioaccumulation and biomagnification. The biggest step in biomagnification occurs from water to algae, where methylmercury concentrations can increase from 100,000 to 1,000,000 times. In contrast, increases for subsequent steps in the food chain (from algae to macroinvertebrates, from macroinvertebrates to forage fish, and from forage fish to predator fish) are individually much smaller (5 to 10 times for each step).
Among streams with large gradients in aqueous methylmercury concentrations, the amount of methylmercury that is available for uptake by algae at the base of the food web (rather than trophic level) can be the most important factor controlling methylmercury concentrations in predator fish. For example, trout in Oregon streams (where aqueous methylmercury concentrations were low) actually had lower mean methylmercury tissue concentrations than macroinvertebrates in Florida streams (where aqueous methylmercury concentrations were high).

Methylmercury in macroinvertebrates and fish correlate positively with concentrations of filtered total and methylmercury in stream water; with concentrations of dissolved organic carbon and its chemical form; and with the extent of wetlands within a basin. These factors likely control the supply of aqueous methylmercury to the base of food webs. Food web structure becomes more important when comparing bioaccumulation among streams whose source loads and environmental settings are similar (such as for streams in the same watershed).

Short answer: Decisions by the EPA regarding a new national mercury emissions regulation will be significantly aided by the improved scientific understanding provided by this study of how mercury sources, watershed cycling, and stream-based food webs interact. Previous to this study, a very limited number of studies had delved into the details of what controls mercury contamination levels in stream ecosystems.
Long answer: Section 112 of the 1990 Clean Air Act Amendments (CAAA) identify seven priority air pollutants, of which mercury is one, and require the EPA to identify the sources of 90% of each pollutant and subject these sources to maximum achievable control technologies. Current considerations for mercury by the EPA are specific to coal and oil fired electric utilities. Our study results relate to CAAA Section112(d)(2) which specify that "any non-air quality health and environmental impacts" can be considered before making a determination on standards for new or existing sources. Our study results provide many new insights into factors regulating mercury contamination levels of stream-based food webs, including the importance of methylmercury sources within watersheds. Last, the results from this study support the notion that not all locations are equal in terms of how they respond to mercury loads and changes to mercury loads. It is important for decision makers to realize that different watersheds, and often different areas within the same watershed, may respond differently to changes in atmospheric mercury loads.